Microcavity semiconductor lasers recently have been described for operation. Such microcavity semiconductor lasers employ an active lasing medium that support optical modes in a short cavity. For example, a microdisk laser is described by McCall et al. in "Whispering-Gallery Mode Microdisk Lasers" in Appl. Phys. Lett. 60, (3), 20 Jan., 1992. Such microcavity laser supports the electromagnetic field mode in a thin disk on top of a small supporting pillar.
Microcavity semiconductor lasers are advantageous as compared to conventional semiconductor lasers in being much smaller in size and requiring substantially less minimum operating current (power) in the range of microwatts. Recently, in Directional Light Coupling From Microdisk Lasers, Appl. Phys. Lett. 62, 561 (1993) an asymmetric point was introduced into a microdisk to provide a location of increased quantity of lasing light to leak out from the point of asymmetry. Light emitted from such a thin disk with thickness d from the asymmetric point can undergo a large angle of diffraction in the directions perpendicular to the disk plane. According to the physical law of diffraction, the diffraction angle is given by Theta=2*ArcTan(lambda/d) in radians and will occur at a distance of 2.pi.d.sup.2 /lambda away from the edge of the disk. The thin disk of thickness around 0.2 micron emitting at a wavelength of 1.5 microns will give rise to a diffraction angle of Theta=2.88 radians or almost 165 degrees and will occur at 0.15 micron from the disk's edge. This means that the light emitted from the disk will disperse away rapidly within a short distance of less than two tenths of a micron, which makes it very difficult, if not impossible, to collect a useful fraction of the output laser light into a semiconductor waveguide in practice.
Microdisk lasers can have high efficiency (with a spontaneous-coupling factor of larger than 0.1) only in the limit of small disk radius of 1-1.5 microns for the emission wavelength of 1.5 microns. The required disk radius scales linearly with the optical wavelength. This makes it very difficult to realize an efficient microdisk laser at short wavelength range, such as the visible wavelength of 0.5 microns. Visible microcavity lasers are important as they have applications to color display or high-density optical storage or sensing. According to the above scaling rule, at the visible wavelength of 0.5 microns, an efficient microdisk will need a disk radius of 0.3-0.5 microns and will be difficult to fabricate and suspend on a pillar.
The difficulty of obtaining useful light from microdisk lasers and their small disk size needed for high cavity efficiency make it difficult to use microdisk lasers in many practical applications.
An object of the present invention is to provide a photonic-wire light emitting device or laser which has an extremely high spontaneous-emission coupling efficiency or factor of for example 0.3 and larger by virtue of use of a photonic-wire waveguide core combined with microcavity structure and which can be scaled to operate at the visible wavelength range without the aforementioned problem.
Another object of the present invention is to provide a photonic-wire microcavity light emitting device or laser amenable for coupling light out from the device to, for example, a semiconductor waveguide efficiently because of its extremely high spontaneous-emission coupling efficiency or factor. In addition, the photonic-wire laser can be modulated at a very high modulation rate, which will enable high data transmission in a photonic circuit.
Yet still another object of the present invention is to provide a photonic-wire microcavity light emitting device or laser amenable for coupling light out from the device to a strongly-guided semiconductor waveguide, making it amenable for coupling to very-high density integrated optical circuits connected by such waveguides. Such strongly-guided waveguides can make bends of smaller than 1 micron radius with negligible photon loss, making it possible to integrate 1000 or more optical components within a 1 millimeter square area and resulting in very-high density photonic integrated circuits. Furthermore, the photonic-wire device can be integrated via direct fabrication on a wafer instead of via hybrid integration. Such very-high density photonic integrated circuits will have applications to optical communications, optical interconnects, optical sensing, optical signal processing, and optical computing.
In comparison, conventional integrated optical circuits are typically made up of hybrid optical components consisting of semiconductor lasers with long cavity lengths of 300 microns (0.3 millimeters). The laser components are coupled to weakly-guided ridge waveguides. A weakly-guided waveguide cannot make a bend with a radius smaller than a few millimeters without incurring very high photon loss because of radiation from the bend. This seriously restricts the integration density of the current typical integrated optical circuits to at most a few components in a millimeter square area.
Yet still an additional object of the present invention is to provide a highly efficient cavity for applications to visible semiconductor laser light sources or light-emitting diodes (LEDs) with edge emission, which are difficult to realize at present due partially to the low efficiency of conventional cavity designs.